The manufacture of semiconductor elements often uses silicon carbide members, for example, wafer boats, mother boats, liner tubes, process tubes, paddles, cantilevers, and forks in heat treatment steps. The semiconductor manufacture places high purity requirements on these silicon carbide members in order to prevent contamination of semiconductor elements. In particular, the heat treatment of semiconductor elements in diffusion furnaces is accompanied by an increased likelihood that semiconductor elements be contaminated by the diffusion of impurities in the members. It is thus very important to prevent impurity diffusion. It has been a common practice to form high purity silicon carbide coatings on the surface of silicon carbide substrates by chemical vapor deposition (CVD) techniques for preventing diffusion of impurities from within the members.
Although the substrate and coating of these silicon carbide members are made of the same material, there is a difference in coefficient of thermal expansion therebetween due to a slight compositional difference therebetween. Since thermal stresses can develop between the substrate and the coating during heat treatment, repeated heat treatments will cause the coating to separate from the substrate. More particularly, silicon carbide substrates are often made of silicon carbide impregnated with metallic silicon, known as SiC-Si, which generally has a coefficient of thermal expansion of 4.3-4.5.times.10.sup.-6 /.degree.C. High purity silicon carbide (SiC) coatings formed by CVD have a coefficient of thermal expansion of 4.5-4.9.times.10.sup.-6 /.degree.C. which is higher than that of the SiC-Si. The differential thermal expansion between SiC-Si substrates and SiC coatings formed thereon by CVD can induce thermal stresses in the SiC coatings during heat treatment. Since thermal cycles are repeated in practice, such repetitive stresses lead to frequent occurrence of cracks and separation. Therefore, members comprising free silicon impregnated silicon carbide substrates having high purity silicon carbide coatings formed by CVD suffer from a relatively short effective life.
Other prior art proposals include a coating having a multilayer structure of silicon carbide (SiC) and silicon nitride (SiN) layers (Japanese Patent Publication No. 45154/1985) and an SiC multilayer structure (Japanese Patent Application Kokai No. 84427/1983). These structures could increase the strength of coatings themselves although no attention was paid to the differential thermal expansion between substrates and coatings, which suggests that their effective life is not satisfactory.